Field effect transistor

ABSTRACT

In a field effect transistor having a quantum well is provided by a primary conduction channel, at least one secondary conduction channel immediately adjacent and in contact with the primary channel has an effect bandgap greater than the effective bandgap of the primary channel, and the modulus of the difference between the impact ionisation threshold IIT of the primary channel and the effective conduction band offset (the height of the step) between the primary and secondary channels being no more than 0.5 Eg (effective), or (alternatively) no more than 0.4 eV. Higher energy carriers which might otherwise cause impact ionization leading to runaway are thus diverted into the secondary channel allowing the device to run faster at increased voltages and/or to exhibit much greater resistance to runaway. The primary channel is prefereably of low bandgap material, for example InSb, InAs, InAs 1-y , In 1-x Ga x As.

This invention relates to field effect transistors (FETs) and inparticular to quantum well field effect transistors, also known asMODFETs (modulation doped field effect transistors) or HEMTs (highelectron mobility transistors).

MODFETs are characterised by a conduction channel of low bandgapmaterial bounded by material with a higher bandgap, so that theconduction channel provides a quantum well region for carrier transportbetween drain and source regions. A gate electrode structure is providedfor controlling the number of current carriers in the conductionchannel, and thus its conductivity. The gate structure may be of ametal-insulator-semiconductor construction, or may comprise a Schottkydiode, for example.

Typical examples of MODFETs are disclosed in European Patent Applicationserial number EP 0523731 (Sumitomo); U.S. Pat. No. 6,100,548 (Nguyen);U.S. Pat. No. 5,856,217 (Nguyen); U.S. Pat. No. 5,334,865 (Fathimulla);U.S. Pat. No. 5,331,185 (Kuwata); U.S. Pat. No. 5,286,662 (Kuwata); U.S.Pat. No. 5,023,674 (Hikosaka); and U.S. Pat. No. 4,710,788 (Dambkes).

In a MODFET the conduction channel possesses an essentiallysingle-crystal lattice so that the carriers have a relatively long meanfree path, and a correspondingly high mobility. The material of theconduction channel is commonly substantially undoped or very lightlydoped to maximise the carrier mobility, velocity and mean free path,although more highly doped materials are sometimes used. The highcarrier mobility in MODFETs renders them particularly suitable forhigh-speed use.

Low band gap materials such as indium arsenide (InAs), indium antimonide(InSb), and indium gallium arsenide (InGaAs) are particularly suitablefor use in MODFETs, with indium antimonide being particularlyadvantageous for ultra high-speed applications since it has low electroneffective mass, very high electron mobility, a large ballistic mean freepath and a high saturation velocity.

FIG. 1 shows in schematic outline form the structure of a MODFET. Lyingbetween conductive doped source and drain regions 3, 4 is a region 2which includes a discrete layer 5 between wider bandgap lower and upperlayers 6, 7 respectively. Layer 5 provides a quantum well region 8 andin practice it is conventionally grown epitaxially on the layer 6 so asto form a single crystal lattice. Source, gate and drain electrodes 9,10, 11 are provided over the corresponding regions 3, 2, 4.

The material(s) of the substrate or layer 6 and that of layer 7 havelarge bandgaps, while the material of layer 5 has a low bandgap, therebyforming with layers 6 and 7 a quantum well heterostructure for currentconduction. Layer 7, and often also layer 6, is doped to supply ortransmit carriers by modulation doping to the layer 5 providing the wellregion 8. The material of layer 5 is preferably undoped, or has verylittle doping, although some quantum well transistors do employconduction channels with relatively highly doped materials.

The corresponding bandgap distribution along the line A—A of FIG. 1 isshown schematically in FIG. 2, where the upper line 12 denotes the edgeof the conduction band and the lower line 13 denotes the edge of thevalence band. The material of layer 5 has a narrow band gap 14. Thelayers 6 and 7 are of the same material and their bandgap 15 issufficiently large to ensure that carriers therein will normally neverattain sufficient energy to enter its conduction band, so carriersremain confined in the well. Thus layer 5 defines with the surroundingwide bandgap layers 6, 7 a quantum well region 8 of width w. It will beobserved that since the quantum well region 8 in this case comprises asingle uniform conducting layer 5, the sides 16 of the well are ideallysubstantially vertical. A dotted line 17 shows the height of the impactionisation threshold within the quantum well. The impact ionisationthreshold (not shown) for layers 6 and 7 lies above the edge 12 of theirconduction bands.

Although it is possible by varying the composition at the edges of thewell region 8 to produce quantum wells with non-vertical sides, so thatthe bandgap changes in a more continuous manner, in the presentinvention it is preferred to have a sharp change in bandgap at the wellsides.

In operation of the quantum well FET of FIG. 1, when the bias applied tothe gate electrode 10 is such that the transistor is on, the populationof carriers in the quantum well region 8 is sufficient that current willflow in the conduction channel if an appropriate potential differenceV_(DS) is applied between the drain and source electrodes 9, 11. Thecurrent will dependent on the magnitude of V_(DS), but as the value ofV_(DS) reaches a threshold level the energy of the carriers in the wellreaches the impact ionisation threshold 17 and additional carriers beginto be created. If the value of V_(DS) rises beyond this point asignificant number of additional current carriers are generated,eventually leading to avalanche breakdown. In this process, all thecarriers continue to be confined in the quantum well region 8 because ofthe high bandgap 15 presented by the material of adjacent layers 6, 7,although escape of the carriers into the barriers is possible if asufficiently high bias is applied.

This is illustrated in FIG. 3, which shows a graph of drain currentI_(D) as a function of drain voltage V_(D), notionally divided intothree regions 18 to 20 separated by knees. As the drain voltage V_(D) isincreased, an initial region 18 (called the “linear region”) is followedby a region 19 of relatively low slope, where the rise in drain currentwith drain voltage is small. Region 19 is called the “saturationregion”, and is the region of normal operation. As the current increasesstill further impact ionisation generates additional carriers untilregion 20 is reached where avalanche breakdown commences and the currentrises more steeply, to an extent which may damage the device. The upperlimit of region 19 is therefore limited by the onset of avalanchebreakdown.

Avalanche breakdown not only leads to loss of control of the transistorin response to the gate bias voltage, but it can produce thermalrunaway, threatening the device and possibly associated components.

Avalanche breakdown is a phenomenon common in many types ofsemiconductor device, and indeed is used to good effect in devices suchas Zener diodes. However, it does give rise to particular problems inthe context of MODFETs which employ low bandgap materials with highcarrier mobilities such as indium antimonide. Because the impactionisation threshold is essentially the material bandgap, in low bandgapmaterials the fields from relatively low voltages V_(DS) give rise toavalanche breakdown. The low threshold value of V_(DS) which producesbreakdown with such low bandgap materials is highly undesirable and atpresent is a significant limitation in high frequency, high voltageapplications.

Furthermore, even at lower energy levels impact ionisation in fieldeffect transistors can gives rise to the kink effect, where holes gatherunder the source, see for example Armstrong et al, Solid StateElectronics, Vol. 39(9), p 1337, 1996. The kink effect increases theoutput conductance, limiting the capacity to drive further devices evenunder more moderate operating conditions.

It will therefore be understood that in field effect transistors thereis a trade off between switching speed—which requires high carriervelocities and mobilities and hence a lower band gap, and breakdownvoltage—which requires a higher band gap. Thus a problem exists when itis necessary to drive an FET with a relatively high voltage for poweringsubsequent stages (e.g. in systems such as modulators and amplifiers, orFET-based logic) while retaining a useful high frequency gain. Suchconsiderations are of particular importance for example in mobilecommunications devices where high power high frequency signalamplification is required for radio frequency signal transmission. Thereis therefore a requirement for high frequency field effect transistorsoperable at relatively high voltages.

In a quantum well field effect transistor according to the presentinvention the quantum well is provided by a primary conduction channeland at least one secondary conduction channel immediately adjacent andin contact with the primary channel, the secondary channel having abandgap greater than that of the primary channel. According to theinvention the conduction band of the secondary channel is close to theimpact ionisation threshold IIT of the first channel. As discussed inmore detail below, preferably the modulus of the difference between theimpact ionisation threshold IIT and the effective conduction band offsetΔE_(C) (effective) between the primary and secondary channels being nomore than 0.5 Eg (effective). Alternative preferences are for themodulus of the difference between the impact ionisation threshold IITand the effective conduction band offset ΔE_(C) (effective) between theprimary and secondary channels to be no more than 0.4 eV, or no morethan the lower of 0.5 Eg (effective) and 0.4 eV.

Composite quantum well channel FETs are known, examples being disclosedin, for example, European Patent Application No. 1030371 (Sumitomo);Japanese Patent Application No. 9283745 (Oki) and the article “DesignCharacteristics of InGaAs/InP Composite Channel HFETs”, Takatomo Enokiet al, IEE Trans. Electron Devices, 45(8), August 1995. However, in noneof these is the impact ionisation threshold IIT close to the effectiveconduction band offset ΔE_(C) (effective) between the primary andsecondary channels.

It will be appreciated that the energies of the carriers are spread overa range, and that initially there will be relatively few carriers ofsufficiently high energy to cross the impact ionisation threshold, thenumber of such carriers increasing with applied voltage (potentialdifference). In a construction according to the present invention, it isbelieved that at least some of the carriers which would otherwise reachthe impact ionisation threshold of the first channel are diverted to asecondary conduction channel, and impact ionisation and the tendency torunaway are accordingly reduced.

Since the secondary conduction channel commonly will not show the sameadvantageous characteristics of the primary channel, for exampleswitching speed, it will be understood that the precise choice of energylevels for a particular transistor according to the invention willrepresent a trade-off between speed and the onset of significant impactionisation. Nevertheless, compared with a prior art device having nosecondary channel it is possible to obtained an improved switchingspeed, by increasing the applied voltage, for the same degree of impactionisation, and/or to obtain a reduction in susceptibility to impactionisation, e.g. at a voltage where impact ionisation in the prior artdevice has become unacceptable.

FIG. 9 illustrates energy levels which might occur in the quantum wellregion of a transistor according to the invention. Electrons in aprimary conduction channel 35 cannot occupy a level lower than the firstsub-band 34 (the Fermi level) of that channel, which lies an amount E₁above the energy zero of that channel. Similarly electrons in anadjacent secondary conduction channel 37 cannot occupy a level lowerthan the first sub-band 36 of channel 37, an amount E₁′ above the energyzero thereof. Thus the effective bandgap in region 35 is given by Eg(effective)=Eg+E₁.

Preferably the material of the conduction channel has an effectivebandgap Eg no greater than 0.75 eV, more preferably no greater than 0.6eV, even more preferably no greater than 0.5 eV, and most preferably nogreater than 0.4 eV.

The effective conduction band offset between the primary and secondarychannels is given by ΔE_(C) (effective)=ΔE_(C)+E₁′−E₁, where ΔE_(C) isthe difference between the absolute energy zeroes of the channels. Theeffective impact ionisation threshold IIT (effective)=Eg+E₁=Eg(effective). Later references in the specific description to energylevels and differences should be read as to the effective values.

Particularly because of the trade-off in performance and susceptibilityto impact ionisation it is presently preferred to arrange for ΔE_(C)(effective) to be relatively close to the impact ionisation thresholdIIT (effective). In particular it is preferred that the differencebetween IIT (effective) and ΔE_(C) (effective) is no more than 0.5 Eg(effective), more preferably no more than 0.25 Eg (effective), even morepreferably no more than 0.125 Eg (effective), and most preferably nomore than 0.05 Eg (effective).

The difference IIT (effective)−ΔE_(C) (effective) may be positive, inwhich case higher energy carriers with an energy less than IIT will bediverted to a secondary channel with lower performance but improvedavoidance of impact ionisation; or the difference may be negative inwhich case some impact ionisation may occur prior to occupation of asecondary channel, but only to an acceptable extent and with filler useof the superior characteristics of the primary channel. As impliedabove, at present it is believed that a substantial matching of IIT(effective) and ΔE_(C) (effective) is a good, or the best, compromise.Thus an alternative way of defining a preferred value of ΔE_(C)(effective) is to say that the difference between IIT (effective) andΔE_(C) (effective) is no more than 0.4 eV, more preferably no more than0.3 eV, more preferably no more than 0.2 eV, and most preferably no morethan 0.1 eV. Again the sign of the difference will affect performance asoutlined above.

While the invention covers transistors where only one secondary channelis provided, preferably on the opposite side of the primary channel froma gate of the transistor, the provision of two secondary channelsprovides further space for high energy carriers to occupy. In one formof embodiment, the two secondary conduction channels are of equalthickness.

Further features and advantages of the invention will become clear tothe reader upon a perusal of the appended claims, and upon a reading ofthe following more detailed and specific description of embodiments ofthe invention, made with reference to the accompanying figures, inwhich:

FIG. 1 is an idealised schematic vertical cross section through a priorart quantum well FET;

FIG. 2 is a bandgap energy diagram for the FET of FIG. 1 taken along theline A—A of FIG. 1;

FIG. 3 shows a graph of drain current I_(D) as a function of drainvoltage V_(D) of a conventional FET illustrating avalanche breakdowneffects;

FIG. 4 is an idealised schematic partial vertical cross section througha first outline embodiment of quantum well FET according to theinvention;

FIG. 5 is a bandgap energy diagram for the FET of FIG. 4 taken along theline B—B;

FIG. 6 shows a graph of drain current I_(D) as a function of drainvoltage V_(D) of the FET of FIG. 5 illustrating the suppression ofavalanche breakdown;

FIG. 7 is an idealised schematic partial vertical cross section throughthe well region of a second embodiment of a quantum well FET accordingto the invention, in more detail than that of FIG. 4;

FIG. 8 is a bandgap energy diagram for the FET of FIG. 7 taken along theline C—C; and

FIG. 9 is a diagram illustrating the effective bandgap in a quantum wellwith quantisation effects included.

Where appropriate in the drawings like references have been used forlike features.

FIG. 4 shows a schematic outline vertical cross section through an FETgenerally similar to that of the FET of FIG. 1 but constructed accordingto the invention. It differs in that the quantum well region 8 is nowcomposed of a plurality of layers 21, 22, 23 of different materials asopposed to the single homogeneous layer 5 of FIG. 2. The central layer22 is similar to the layer 5 of FIG. 2, and provides a primaryconduction channel, but it is now bounded on either side by layers 21,23 which provide secondary conduction channels. These are formed of amaterial having a greater bandgap than layer 22 with a conduction bandedge which is approximately equal in energy to the impact ionisationthreshold of layer 22. The bandgap of the material of layers 21, 23 isless than that of the layers 6, 7. Optionally, the electrode layer 10 isunderlain by a dielectric layer, or is in the form of a Schottky diodein known manner.

Preferably the layers 21, 23 are (a) of equal thickness, (b) of the samematerial and (c) have the same bandgap; however, none of these featuresis strictly necessary.

The bandgap distribution along the line B—B of FIG. 4 is shown in FIG.5. The material of layers 21, 23 has a bandgap 24 with a conduction bandcommencing at a level 26, the magnitude of both the bandgap and theconduction band edge being intermediate those of the central layer 22and the widest bandgap material of the layers 6, 7. The impactionisation threshold of layers 21, 23 is shown at 28.

It will be seen that the quantum well region 8 thus defined comprises aprimary conduction channel 27 and adjacent secondary conduction channels25. For an InSb-based FET, with no strain and quantisation effectsincluded, the bandgaps 14, 24 and 15 typically have values of 0.178,0.445 and 0.773 eV respectively. With strain and quantisation effectsincluded, the effective bandgaps 14, 24 and 15 typically have values of0.220, 0.559 and 0.872 eV respectively for a 20 nm wide primary well.

In use, the carriers are initially essentially confined to the primaryconduction channel 27. As the potential difference between the sourceand drain electrodes is increased, their energy of carriers approachesthe impact ionisation threshold 17. However, once the energy exceeds thelevel of the conduction band 26, the carriers can also occupy thesecondary channels 25, which have a higher impact ionisation threshold.The result is a reduction in impact ionisation. Where two channels 25 ofwidth equal to the primary channel 27 are present, the reduction is afactor of approximately three. As in FIG. 2, the high bandgap layers 6,7 continue to confine the electrons within the quantum well region 8.

While the secondary channels 25 are made from higher bandgap material,and therefore have a rather lower electron mobility relative to theprimary channel, they can still have a good electron velocity. Since atlow field all the electrons remain in the primary channel 27, the lowerfield mobility in channels 25 is not so important to the efficientfunctioning of the transistor.

Although it is preferred, it is not necessary to have a secondarychannel on each side of the primary channel. One or other of the layers21, 23 may be omitted, in which case the well is preferably locatedbetween the remaining secondary channel and the gate. However with asingle secondary channel there is a corresponding increase in impactionisation.

The energy level plot is not necessarily symmetrical about the quantumwell. Thus in FIG. 5 for example the two levels 26, although shown asbeing equal, may be different, so that the quantum well effectivelywidens on one side and then the other with increasing carrierexcitation.

Furthermore, between any secondary channel and the substrate orsubstrate layer there may be provided one or more (successive) tertiarychannels with increasing conduction band levels, etc., so that they actin a manner similar to that of the secondary channel in permitting thequantum well to become increasingly wide with increasing excitation ofthe carriers. Again the additional energy levels thus provided can bethe same or different when there are tertiary channels provided on bothsides of the quantum well.

The plot of I_(D) against V_(D) for a transistor according to theinvention is shown in FIG. 6. Following the initial linear region 29 ofrelatively high slope the saturation region 30 extends to higher valuesof V_(D) than in the prior art device, due to the reduction in impactionisation, and leading to a larger range of operation.

Although the basic invention is described above, further improvementsand advantages can be envisaged. For example, placing the channel dopingatoms in the surrounding channel areas 21, 23 will tend to reduce themobility here, providing a negative channel conductance effect whichwill further counteract the increase in channel conductance due to theimpact ionisation, and making devices with even harder (i.e. lowerslope) output characteristics.

FIG. 7 illustrates the well region of a transistor which is similar tothat of FIG. 4, but modified in that the high bandgap layers arerestricted in thickness and bounded on their outer surfaces by lowerbandgap 33 regions 31, 32. By way of example only, the regions 31 and 32may be made of the same materials as that of regions 21 and 23, with thesame bandgap. The corresponding bandgap distribution for such an exampleis shown in FIG. 8. For an InSb-based FET with no strain or quantisationeffects included the bandgaps 14, 15, 24 and 33 typically have values of0.178, 0.773, 0.445 and 0.445 eV respectively. With strain andquantisation effects included, the effective bandgaps 14, 15, 24 and 33typically have values of 0.220, 0.872, 0.559 and 0.559 eV respectivelyfor a 20 nm wide primary well.

In one example of a well region 8 of an FET according to the inventionand as shown schematically in FIG. 7, the 200 Å thick central primaryconduction channel 27 (22) is of undoped indium antimonide, and isbounded by 200 Å thick secondary conduction channels 25 (23, 23) ofIn_(0.85)Al_(0.15)Sb, which also provides the outermost regions 31, 32.The 200 Å thick high bandgap regions 6, 7 are of In_(0.70)Al_(0.30)Sb.This then provides a structure that is strain balanced at the latticeconstant of the In_(0.85)Al_(0.15)Sb. It should be noted that in allcases the layers are nominally undoped, but may contain unintentionaldoping of either type. The channel doping may be provided by δ-dopinglayers placed above and/or below the central well, or by doping any partof the structure n-type.

When holes are generated in the impact ionisation process, they tend tocollect under the source, thereby producing the kink effect mentionedabove, and which commonly occurs in narrow bandgap devices. This effectmay be alleviated or avoided in transistors according to the inventionfor example by confining the holes in a valence band so that they areremoved at the source contact and/or by providing a back contactarranged so that the holes will move preferentially towards the back ofthe transistor for removal.

1. A quantum well field effect transistor wherein the quantum well isprovided by a primary conduction channel and at least one secondaryconduction channel adjacent and in contact with the primary channel, thesecondary channel having an effective bandgap greater than the effectivebandgap Eg (effective) of the primary channel, wherein the modulus ofthe difference between the effective impact ionisation threshold IIT(effective) of the primary channel and the effective conduction bandoffset ΔE_(c) (effective) between the primary and secondary channels isnot more than 0.5 Eg (effective).
 2. A transistor according to claim 1wherein the modulus is no more than 0.25 Eg (effective).
 3. A quantumwell field effect transistor wherein the quantum well is provided by aprimary conduction channel and at least one secondary conduction channeladjacent and in contact with the primary channel, the secondary channelhaving an effective bandgap greater than the effective bandgap Eg(effective) of the primary channel, wherein the modulus of thedifference between the effective impact ionisation threshold IIT(effective) of the primary channel and the effective conduction bandoffset ΔE_(c) (effective) between the primary and secondary channels isnot more than 0.25 Eg (effective).
 4. A transistor according to claim 3wherein the modulus is no more than 0.3 eV.
 5. A transistor according toclaim 3 wherein the difference is positive.
 6. A transistor according toclaim 3 wherein the bandgap of the material of the primary channel isless than 0.75 eV.
 7. A transistor according to claim 3 wherein thesecondary conduction channel is on each side of the primary channel. 8.A transistor according to claim 3 having a gate on the opposite side ofthe primary channel from the secondary conduction channel.
 9. Atransistor according to claim 3 wherein the quantum well furthercomprises at least one tertiary channel, the at least one secondarychannel being located between and in contact with the primary channeland the tertiary channel, the tertiary channel having an effectivebandgap greater than that of the secondary channel with which it is incontact and a conduction band edge approximately equal in energy to theimpact ionisation threshold of the secondary channel with which it is incontact.
 10. A transistor according to claim 3 wherein the material ofthe primary channel has an effective conduction band offset ΔE_(c)(effective) which is equal or greater than its effective bandgap Eg(effective).
 11. A transistor according to claim 3 where the primarychannel is formed from InSb, InAs, InAs_(1-y)Sb_(y), In_(1-x)Ga_(x)Sb orIn_(1-x)Ga_(x)As.
 12. A transistor according to claim 3 wherein channeldoping is provided in the secondary channel.
 13. A transistor accordingto claim 3 and constructed so that holes are confined in a valence bandwell for removal at the source contact.
 14. A transistor according toclaim 3 and provided with a substrate contact for removal of holes. 15.A quantum well field effect transistor wherein the quantum well isprovided by a primary conduction channel and at least one secondaryconduction channel immediately adjacent and in contact with the primarychannel, the secondary channel having an effective bandgap greater thanthe effective bandgap Eg (effective) of the primary channel, wherein themodulus of the difference between the effective impact ionisationthreshold IIT (effective) of the primary channel and the effectiveconduction band offset ΔE_(c) (effective) between the primary andsecondary channels is not more than 0.4 eV.
 16. A transistor accordingto claim 15 wherein the modulus is no more than 0.3 eV.
 17. A transistoraccording to claim 15 wherein the difference is positive.
 18. Atransistor according to claim 15 wherein the bandgap of the material ofthe primary channel is less than 0.75 eV.
 19. A transistor according toclaim 15 wherein the secondary conduction channel is on each side of theprimary channel.
 20. A transistor according to claim 15 having a gate onan opposite side of the primary channel from the secondary conductionchannel.
 21. A transistor according to claim 15 wherein the quantum wellfurther comprises at least one tertiary channel, the at least onesecondary channel being located between and in contact with the primarychannel and the tertiary channel, the tertiary channel having aneffective bandgap greater than that of the secondary channel with whichit is in contact and a conduction band edge approximately equal inenergy to the impact ionisation threshold of the secondary channel withwhich it is in contact.
 22. A transistor according to claim 15 whereinthe material of the primary channel has an effective conduction bandoffset ΔE_(c) (effective) which is equal or greater than its effectivebandgap Eg (effective).
 23. A transistor according to claim 15 where theprimary channel is formed from InSb, InAs, InAs_(1-y)Sb_(y),In_(1-x)Ga_(x)Sb or In_(1-x)Ga_(x)As.
 24. A transistor according toclaim 15 having doping in the secondary channel.
 25. A transistoraccording to claim 15 having a source contact and constructed forconfining holes in a valence band well for removal at the sourcecontact.
 26. A transistor according to claim 15 having a substratecontact for removal of holes.